As containers for containing beverages and foods, there have heretofore been widely used cup-type containers comprising a thermoplastic resin and having a body portion hanging down from the inner edge of the flange and a bottom portion.
A variety of methods have been proposed for forming the cup-type containers, such as those methods that are usually based on the injection forming or the pressure forming. There has, further, been proposed a forming method based on the compression forming (patent document 1).
However, a thin cup-type container made from a polypropylene by injection forming or pressure forming has a high degree of anisotropy in the molecular orientation of the container, and cannot provide a sufficient degree of heat resistance and is not, either, still satisfactory from the standpoint of gas barrier property of the container.
Even if the container is formed by the compression forming described in the above patent document 1, the space for forming the side wall is limited earlier than the flow of the resin; i.e., the resin must flow through the limited space. Therefore, anisotropy occurs in the molecular orientation, and a satisfactory heat resistance is not obtained.
Besides, forming the articles by the injection forming or the pressure forming is necessarily accompanied by the generation of scrap resins. Therefore, it has been desired to efficiently form the polypropylene cups without developing scrap resins.
In order to solve the above problems, there have been proposed cup-type containers made from a polypropylene by the compression forming, the cup-type containers being uniformly oriented in the direction of height of the container and in the circumferential direction thereof, and having a uniform strength in all directions and improved mechanical strengths such as shatter strength, shock resistance and strength against the pressure (patent document 2). In the cup-type containers formed by the compression forming, the problem of anisotropy in the molecular orientation has been solved, and attention has been given to the β-crystals in the crystal structure of the polypropylene. Namely, the β-crystals are made present in at least locally in the body portion making it possible to impart excellent heat resistance to the cup-type containers.